Organic electroluminescent devices, also known as organic light emitting diodes (OLEDs), are a type of devices which can directly convert electric energy to light energy. In 1987, C. W. Tang and collaborators first prepared a small-molecular organic electroluminescent device having a double-layer structure by using a hole transport material N,N′-diphenyl-N,N′-bis(3-tolyl)-4,4′-benzidine as a hole transport layer, and 8-hydroxyquinoline aluminum having electron transport capacity as an electron transport layer and a light-emitting layer.
Organic electroluminescence may involve the following five processes: 1) injection of carriers, during which electrons and holes are injected from the cathode and the anode, respectively, to the organic functional film layer arranged between the electrodes under the effect of an applied electric field; 2) migration of carriers, during which the injected electrons and holes migrate to the light emitting layer from the electron transport layer and the hole transport layer, respectively; 3) recombination of carriers, during which electrons and holes combine in the light-emitting layer to produce excitons; 4) migration of excitons, during which excitons migrate under the effect of the electric field, transfer their energy to the luminescent molecules, and excite electrons to transit from ground state to excited state; and 5) electroluminescence, during which the excited state is deactivated through energy irradiation to generate photons and release energy.
In these five processes, the energy-level match between the layers is very important, which directly affects the ultimate performance of the device. The HOMO value of the hole transport layer material is somewhat different from that of the anode material and the anode may release oxygen after long-time operation, which destroys the organic layer and results in dark spots. Therefore, a hole injection layer, which has a HOMO value between those of the anode and the hole transport layer, is normally inserted between the anode and the hole transport layer to facilitate hole injection. In addition, its film characteristics enable it to prevent oxygen in the anode from entering the OLED device, so as to prolong the life time of the device. Furthermore, the hole injection layer can also increase the adhesiveness of the hole transport layer and the anode, increase the hole injection contact, balance electron and hole injections, increase the proportion of exciton production from electron and hole, and control or reduce the number of holes which do not participate in light emission, thereby increasing the efficiency of the device. In conventional organic electroluminescent devices, due to the low contact area between the hole injection layer and an adjacent layer, the contact is poor, which results in reduction of hole injection efficiency, thereby affecting the luminescent efficiency and the luminescent brightness of the organic electroluminescent device.